This invention relates generally to a small volume electron capture detector cell and in particular to methods of minimizing or eliminating mixing effects in such a cell.
By the electron capture detection technique in the active volume of gas chromatography, a tritium or Ni.sup.63 source ionizes the molecules of a carrier or make-up gas as it flows through the detector and the slow electrons thus produced are caused to migrate to the anode, forming a steady or pulsed current. This current becomes reduced if a sample containing electron absorbing molecules is introduced and this loss of current can be amplifed by an electrometer for analysis.
The electron capture detector is extremely sensitive to certain molecules such as alkyl halides, but is relatively insensitive to hydrocarbons, alcohols, ketones, etc. This selective sensitivity to halides makes the detection method especially valuable for the trace analysis of many environmentally important organic compounds, such as pesticides. Electron capture detectors, however, have not been used extensively in conjunction with high resolution capillary columns. They have often been considered to be too large in volume to be suitable for use with high resolution systems, and the detector cell generally contained regions not actively swept by the carrier gas. The latter phenomenon is sometimes called the mixing effects, or mixing volume effects, and such unswept regions are known to cause tailing of chromatographic peaks. The degree to which mixing occurs within the cell is dependent upon cell design and gas flow rate, this problem generally increasing as the length-to-diameter ratio (L/D) of the cell decreases.
It is therefore a general object of this invention to provide an electron capture detector suitable for high resolution analysis.
It is another object of the present invention to provide a small volume electron capture detector cell wherein the mixing volume effects are minimized or eliminated.
It is a further object of the present invention to provide an electron capture detector cell having an insert for eliminating the unswept regions therein.